A number of enzymatic processes have been identified in recent years which act to repair lesions in DNA induced by alkylating agents. These enzymes include the glycosylases, Apurinic/Apyrimidinic endonucleases and the methyltransferases. The biological consequences of defects in these enzyme systems are for the most part unknown. Few mutants have been identified which are defective in these processes. Two methods will be used to select for mutants defective in repair of alkylation damage to DNA. One will be selective for mutants unable to repair Lambda bacteriophage treated with alkylating agents. The second will be to mutagenize with the insertion mutagen Mu d(Ap lac) and screen directly for sensitive mutants. By these methods, we expect to accumulate strains containing mutations in the enzymes responsible for repair of alkylated DNA as well as mutations in genes which regulate these enzymes or are required for their expression. Such mutants will permit the systematic evaluation of the relative roles these repair enzymes perform in the recovery from alkylation damage, evaluation of the mutagenic properties of the lesions upon which specific enzymes act and identification of the genes which code for these enzymes.